Whitelist pdf viewer extension to access chrome resources

There were CORS problems when accessing the chrome resources from
the pdf viewer extensions when coming from a blank page. It should
be fine to access the chrome resources from the builtin extension.

Fixes: QTBUG-75127
Change-Id: I28f981b4ff1d2db86a62507eb25cae45258e3bda
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>